In recent years, the use of high strength-to-weight ratio fiber-reinforced resin composites has continuously expanded Parts or articles made from layers or plies of these fiber-reinforced resin composites are generally created by forming a laminate or lay-up, e.g., a stacking of individual layers of the desired composite material. Each layer or ply of the composite material is typically formed from unidirectional or multidirectional woven fabrics made of glass, graphite fibers, or other fibers which are impregnated with a resin. Such pre-impregnated plies are commonly referred to as "prepreg" plies or simply prepreg. Normally the lay-up is built atop a forming or lay-up tool, which, in its simplest form, may comprise a flat working surface.
Each of the composite material layers is generally cut to a desired shape and placed on top of the previously cut layer or stack of layers to form a lay-up. Thus, the lay-up is a laminate of layers which have been stacked to form the desired three dimensional object or part. After the complete lay-up is prepared, heat and pressure are applied. The heat cures the resin and the pressure compresses the lay-up preventing air and other gases, including volatile gases, from forming porosity or bubbles in the resin as it cures. Normally, an autoclave is used to apply the necessary heat and pressure, yielding the finished article. Alternatively, the cure is accomplished under complete or partial vacuum, with heat.
This fiber/polymer composite prepreg material is generally shipped from a material supplier to a composite products manufacturer in large rolls. Each roll generally includes a plastic separator film or ply (interleaf) which is attached to the generally continuous composite layer to prevent the composite layer from sticking to itself when the material is initially rolled as well as unrolled. This same kind of separator film is used for prepreg material shipped as flat sheets, such as for boron-epoxy. Also, when the product or lay-up is being built, the separator film serves to protect the lower surface of each composite layer from contamination while being cut, formed or otherwise worked. Once each of the layers has been cut to its desired shape, or otherwise formed, the separator film is removed and the layer is transferred to the lay-up tool.
In a similar fashion, modern adhesive films are generally supplied in rolls, or alternatively in sheets A number of these modern adhesive films have a resin base that is similar to the resin used in composite prepreg materials. These adhesive films are generally tacky and require a separator ply or film to prevent self-adhesion and contamination as described above.
The prior art separator films are generally made from an inert material and applied as a very thin ply or film. These separator films are typically made from polyethylene or polypropylene, and discarded as each layer of prepreg or adhesive is laid down or otherwise applied.
A problem occurs when the separator film of the prior art is not removed or not fully removed from the composite layer to which it is attached. When this occurs, the separator film or a portion thereof ends up as an additional layer in the lay-up. Thus, the separator film is embedded within the composite lay-up as it is cured. This separator film acts as a delaminating site within the part, because the resin will not adhere to it, which will then lead to a much weaker part in addition to a part that may not have the expected or desired characteristics. A similar problem occurs when the separator film is not removed from an adhesive film layer.
An additional problem involves detecting any separator film which has been inadvertently left within a lay-up or even within a finished part. The problem of detecting such a separator film is worsened when using automatic lay-up machinery which does not have the capability to detect separator film within the lay-up.
As stated, the separator film does not bond to the resins of most composite materials or adhesive films. However, separator films adhere sufficiently well such that no gaps are created in the lay-ups or even in the cured parts. This makes subsequent detection of any separator film material inadvertently left in a lay-up difficult and unreliable.
Any separator film inadvertently left in the finished part usually results in the need to perform a structural repair of the composite part. This repair is due, in part, to the failure of the separator ply to bond to the matrix resin of most composite materials. These repairs are generally expensive and time consuming, as is the detection of remaining separator film. Further, even if the separator film did or could bond to the matrix resin, the strength and other properties of the separator film may not be adequate or suitable for the desired part. Thus, an intensive inspection as well as an expensive repair is still required.
Alternate configurations of composite prepreg materials includes a thermoplastic material applied to at least one side of each layer. The thermoplastic material and the composite prepreg material generally do not mix or otherwise react. The thermoplastic material acts to toughen the overall lay-up. However, the composite material, including the applied thermoplastic material, is still generally supplied as a roll using a separator film to keep the layers apart. Thus, the problem of the separator film inadvertently left in the lay-up remains.
An alternative method of removing separator film may include chemically decomposing the separator film, such as by a chemical treatment. This chemical decomposing could take place prior to assembly or placement of the composite layer on the lay-up. However, chemical decomposition of the separator film would require that an acidic or basic moiety be present in the separator film which in turn would facilitate the decomposition process. This is particularly true if the separator film is a polyester or polyamide, such as Mylar or nylon. Since the presently used separator film is typically a polyethylene, as mentioned above, this approach is not feasible. Decomposition of these separator films, or any other hydrocarbon or fluorocarbon film, requires strong oxidizing agents and/or very high temperatures. These techniques are not practical since the agents and/or process would adversely affect the properties of the composite layers and the lay-up. Thus, a less stringent process of film degradation and removal is needed.
Additionally, the separator ply material must be non-tacky at room temperature to prevent sticking of the separator film during the rolling up of the composite layer or layers and also during handling, as well as at lower temperatures to prevent similar problems during storage. It must also not adversely affect the primary resin matrix as it cures or afterwards.
Thus, there is a need for a separator film which, if inadvertently left on a layer of prepreg composite material will not require expensive detection analysis. There is a further need for a separator film which, if inadvertently left on a layer of prepreg composite material will not require expensive repairs to the finished part. There is also a need for a separator film which can be purposely left on a prepreg composite layer when forming a lay-up and which may actually enhance the desired properties of the finished part. There is also a need for a separator film having the above properties which can be used with alternative composite material configurations, is generally nontoxic, and inexpensive.